Zinc aluminate spinel impurity phase in Al doped ZnO ceramic target and pulsed laser ablated films: Curse or blessing?

Al doped ZnO films (AZO) have been grown by pulsed laser deposition (PLD) technique using ceramic targets sintered at different temperatures in the range 800-1400 degrees C. The effect of target sintering temperature on the structural, optical and electrical properties of the AZO films has been investigated. The X-ray diffraction (XRD) patterns show that besides the major hexagonal wurtzite phase of ZnO, the zinc aluminate (ZnAl2O4) spinel impurity phase is present predominantly in the targets sintered at 900 degrees C onwards. The XRD peak intensity of the spinel phase increases as the sintering temperature increases. Although, no such impurity has been primarily appeared in the XRD pattern of the films deposited from these targets, the presence of a probable spinel phase has been sensed by the X-ray photoelectron spectroscopy (XPS) measurements. All the films show more than 90% transparency in the region 500-1400 nm. The electrical resistance of the target pellets also decreases with an increase in the sintering temperature up to 1100 degrees C and then increases. Similarly, the carrier concentration increases and the resistivity decreases with the sintering temperature attaining the values of 6.36 x 10(20) cm(-3) and 6.38 x 10(-4) Omega cm respectively for the films ablated from the targets sintered at temperatures 1000 C (AZO1000) and 1100 degrees C (AZO1100) beyond which both the values deteriorate. An increase in the carrier concentration value is assigned to the modification of the grain boundaries by the spinel phase. Vacuum annealing of AZO1000 film at 350 degrees C results in a further increase in the carrier concentration to a value of 1 x 10(21) cm(-3). This film has been applied as a transparent electrode without texturing for a-Si:H solar cell which shows a conversion efficiency of similar to 3.93%. Our study for the first time shows that the presence of a ZnAl2O4 phase to a certain extent in AZO film rather helps to achieve higher carrier concentration. (C) 2014 Elsevier Ltd. All rights reserved.